Endothelium is a target of inflammatory mediators that increase vascular permeability in multiple debilitating lung diseases, including acute respiratory distress syndrome. Increased vascular permeability results from gap formation at sites of endothelial cell junctions. Blocking vascular endothelial cadherin function causes interstitial pulmonary edema, however normal endothelial cell junctions are formed in the absence of vascular endothelial cadherin. This indicates other adhesion molecules play an important role in endothelial cell-cell apposition. However, the molecular anatomy of endothelial cell junctions is poorly understood. Emerging data indicate pulmonary micro-vascular endothelial cells form significantly tighter permeability barriers than do pulmonary artery endothelial cells. This highlights molecular heterogeneity in the lung vasculature that may reveal insight into the anatomy of the endothelial junctions. Indeed, our preliminary studies indicate that CD166/ALCAM, a cell adhesion molecule of the immunoglobulin super-family, is significantly more abundant in pulmonary micro-vascular endothelial cells than in pulmonary artery endothelial cells. Moreover, we discovered ALCAM was enriched in cell junctions in pulmonary micro-vascular endothelial cells in vitro and in vivo, but was virtually absent in pulmonary artery endothelial cells. Further studies revealed co-localization between ALCAM and vascular endothelial cadherin. Blocking ALCAM function resulted in cytoskeletal rearrangement, loss of endothelial adhesion and inhibition of proliferation. Thus, ALCAM and VE-cadherin appear to form a multi-protein complex that controls permeability and proliferation. This proposal therefore tests the OVERALL HYPOTHESIS that ALCAM is a differentially regulated component of the endothelial cell junctions with a dominant role in endothelial permeability and proliferation. Specific Aims test the related hypotheses that: (1) ALCAM is an essential component of the adherens junction in endothelial cells; (2) Endothelial monolayer permeability and proliferation is dominantly regulated by ALCAM. The proposed studies are significant because they may reveal the molecular make-up of lung endothelial cell junctions. If true, then pharmacological agents that modulate the ALCAM-VE-cadherin interaction could be developed to control vascular permeability.